Justin Crocker's reading list...
Wednesday, February 29, 2012
Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes
Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes
Nature 483, 7387 (2012). doi:10.1038/nature10843
Authors: Jennifer F. Hughes, Helen Skaletsky, Laura G. Brown, Tatyana Pyntikova, Tina Graves, Robert S. Fulton, Shannon Dugan, Yan Ding, Christian J. Buhay, Colin Kremitzki, Qiaoyan Wang, Hua Shen, Michael Holder, Donna Villasana, Lynne V. Nazareth, Andrew Cree, Laura Courtney, Joelle Veizer, Holland Kotkiewicz, Ting-Jan Cho, Natalia Koutseva, Steve Rozen, Donna M. Muzny, Wesley C. Warren, Richard A. Gibbs, Richard K. Wilson & David C. Page
The human X and Y chromosomes evolved from an ordinary pair of autosomes during the past 200–300 million years. The human MSY (male-specific region of Y chromosome) retains only three percent of the ancestral autosomes’ genes owing to genetic decay. This evolutionary decay was driven by a series of five ‘stratification’ events. Each event suppressed X–Y crossing over within a chromosome segment or ‘stratum’, incorporated that segment into the MSY and subjected its genes to the erosive forces that attend the absence of crossing over. The last of these events occurred 30 million years ago, 5 million years before the human and Old World monkey lineages diverged. Although speculation abounds regarding ongoing decay and looming extinction of the human Y chromosome, remarkably little is known about how many MSY genes were lost in the human lineage in the 25 million years that have followed its separation from the Old World monkey lineage. To investigate this question, we sequenced the MSY of the rhesus macaque, an Old World monkey, and compared it to the human MSY. We discovered that during the last 25 million years MSY gene loss in the human lineage was limited to the youngest stratum (stratum 5), which comprises three percent of the human MSY. In the older strata, which collectively comprise the bulk of the human MSY, gene loss evidently ceased more than 25 million years ago. Likewise, the rhesus MSY has not lost any older genes (from strata 1–4) during the past 25 million years, despite its major structural differences to the human MSY. The rhesus MSY is simpler, with few amplified gene families or palindromes that might enable intrachromosomal recombination and repair. We present an empirical reconstruction of human MSY evolution in which each stratum transitioned from rapid, exponential loss of ancestral genes to strict conservation through purifying selection.
Conversion of the chill susceptible fruit fly larva (Drosophila melanogaster) to a freeze tolerant organism [Applied Biological Sciences]
Tuesday, February 28, 2012
ChromHMM: automating chromatin-state discovery and characterization
ChromHMM: automating chromatin-state discovery and characterization
Nature Methods 9, 215 (2012).
doi:10.1038/nmeth.1906
Authors: Jason Ernst & Manolis Kellis
Expresso® CMV system: effortless mammalian expression cloning
Expresso® CMV system: effortless mammalian expression cloning
Nature Methods 9, (2012).
doi:10.1038/nmeth.f.356
Authors: Saurabh Sen, Heather Sternard, Eric Steinmetz & Ronald Godiska
Spatiotemporal control of gene expression by a light-switchable transgene system
Spatiotemporal control of gene expression by a light-switchable transgene system
Nature Methods 9, 266 (2012).
doi:10.1038/nmeth.1892
Authors: Xue Wang, Xianjun Chen & Yi Yang
We developed a light-switchable transgene system based on a synthetic, genetically encoded light-switchable transactivator. The transactivator binds promoters upon blue-light exposure and rapidly initiates transcription of target transgenes in mammalian cells and in mice. This transgene system provides a robust and convenient way to spatiotemporally control gene expression and can be used to manipulate many biological processes in living systems with minimal perturbation.
Interplay between gene expression noise and regulatory network architecture
Monday, February 27, 2012
Hsp90 is important for fecundity, longevity, and buffering of cryptic deleterious variation in wild fly populations
In the laboratory, the Drosophila melanogaster heat shock protein Hsp90 can buffer the phenotypic effects of genetic variation. Laboratory experiments either manipulate Hsp90 activity pharmacologically, or they induce mutations with strong effects in the gene Hsp83, the single-copy fly gene encoding Hsp90. It is unknown whether observations from such laboratory experiments are relevant in the wild.
Results:
We here study naturally occurring mutations in Hsp83, and their effects on fitness and phenotypic buffering in flies derived from wild populations. We examined more than 4500 flies from 42 Drosophila populations distributed world-wide for insertions or deletions of mobile DNA in or near the Hsp83 gene. The insertions we observed occur at low population frequencies, and reduce Hsp83 gene expression. In competition experiments, mutant flies performed much more poorly than wild-type flies. Mutant flies were also significantly less fecund and shorter-lived than wild-type flies, as well as less well buffered against cryptic deleterious variation, as we show through inbreeding experiments. Specifically, in Hsp83 mutant flies female fecundity dropped to much lower levels after inbreeding than in wild-type flies. At even slightly elevated temperatures, inbred mutant Hsp83 populations went extinct, whereas inbred wild-type populations persisted.
Conclusions:
Our work shows that Hsp90, a regulator of the stress response and of signaling, helps buffer deleterious variation in fruit flies derived from wild population, and that its buffering role becomes even more important under heat stress.
A non-canonical DNA structure is a binding motif for the transcription factor SP1 in vitro
SP1 is a ubiquitous transcription factor that is involved in the regulation of various house-keeping genes. It is known that it acts by binding to a double-stranded consensus motif. Here, we have discovered that SP1 binds also to a non-canonical DNA structure, a G-quadruplex, with high affinity. In particular, we have studied the SP1 binding site within the promoter region of the c-KIT oncogene and found that this site can fold into an anti-parallel two-tetrad G-quadruplex. SP1 pull-down experiments from cellular extracts, together with biophysical binding assays revealed that SP1 has a comparable binding affinity for this G-quadruplex structure and the canonical SP1 duplex sequence. Using SP1 ChIP-on-chip data sets, we have also found that 87% of SP1 binding sites overlap with G-quadruplex forming sequences. Furthermore, while many of these immuoprecipitated sequences (36%) even lack the minimal SP1 consensus motif, 5'-GGGCGG-3', we have shown that 77% of them are putative G-quadruplexes. Collectively, these data suggest that SP1 is able to bind both, canonical SP1 duplex DNA as well as G-quadruplex structures in vitro and we hypothesize that both types of interactions may occur in cells.
Chromosome-scale selective sweeps shape Caenorhabditis elegans genomic diversity
Chromosome-scale selective sweeps shape Caenorhabditis elegans genomic diversity
Nature Genetics 44, 285 (2012).
doi:10.1038/ng.1050
Authors: Erik C Andersen, Justin P Gerke, Joshua A Shapiro, Jonathan R Crissman, Rajarshi Ghosh, Joshua S Bloom, Marie-Anne Félix & Leonid Kruglyak
Friday, February 24, 2012
Stickleback population genomics
Understanding the genetics of adaptation is a central focus in evolutionary biology. Here, we use a population genomics approach to examine striking parallel morphological divergences of parapatric stream–lake ecotypes of threespine stickleback fish in three watersheds on the Haida Gwaii archipelago, western Canada. Genome-wide variation at greater than 1000 single nucleotide polymorphism loci indicate separate origin of giant lake and small-bodied stream fish within each watershed (mean FST between watersheds = 0.244 and within = 0.114). Genome scans within watersheds identified a total of 21 genomic regions that are highly differentiated between ecotypes and are probably subject to directional selection. Most outliers were watershed-specific, but genomic regions undergoing parallel genetic changes in multiple watersheds were also identified. Interestingly, several of the stream–lake outlier regions match those previously identified in marine–freshwater and benthic–limnetic genome scans, indicating reuse of the same genetic loci in different adaptive scenarios. We also identified multiple new outlier loci, which may contribute to unique aspects of differentiation in stream–lake environments. Overall, our data emphasize the important role of ecological boundaries in driving both local and broadly occurring parallel genetic changes during adaptation.
Review. Robustness in evolution
Phenotypes that vary in response to DNA mutations are essential for evolutionary adaptation and innovation. Therefore, it seems that robustness, a lack of phenotypic variability, must hinder adaptation. The main purpose of this review is to show why this is not necessarily correct. There are two reasons. The first is that robustness causes the existence of genotype networks—large connected sets of genotypes with the same phenotype. I discuss why genotype networks facilitate phenotypic variability. The second reason emerges from the evolutionary dynamics of evolving populations on genotype networks. I discuss how these dynamics can render highly robust phenotypes more variable, using examples from protein and RNA macromolecules. In addition, robustness can help avoid an important evolutionary conflict between the interests of individuals and populations—a conflict that can impede evolutionary adaptation.
Developmental Transcriptional Networks Are Required to Maintain Neuronal Subtype Identity in the Mature Nervous System
by Kevin T. Eade, Hailey A. Fancher, Marc S. Ridyard, Douglas W. Allan
During neurogenesis, transcription factors combinatorially specify neuronal fates and then differentiate subtype identities by inducing subtype-specific gene expression profiles. But how is neuronal subtype identity maintained in mature neurons? Modeling this question in two Drosophila neuronal subtypes (Tv1 and Tv4), we test whether the subtype transcription factor networks that direct differentiation during development are required persistently for long-term maintenance of subtype identity. By conditional transcription factor knockdown in adult Tv neurons after normal development, we find that most transcription factors within the Tv1/Tv4 subtype transcription networks are indeed required to maintain Tv1/Tv4 subtype-specific gene expression in adults. Thus, gene expression profiles are not simply “locked-in,” but must be actively maintained by persistent developmental transcription factor networks. We also examined the cross-regulatory relationships between all transcription factors that persisted in adult Tv1/Tv4 neurons. We show that certain critical cross-regulatory relationships that had existed between these transcription factors during development were no longer present in the mature adult neuron. This points to key differences between developmental and maintenance transcriptional regulatory networks in individual neurons. Together, our results provide novel insight showing that the maintenance of subtype identity is an active process underpinned by persistently active, combinatorially-acting, developmental transcription factors. These findings have implications for understanding the maintenance of all long-lived cell types and the functional degeneration of neurons in the aging brain.
[Report] Evolutionarily Assembled cis-Regulatory Module at a Human Ciliopathy Locus
Authors: Jeong Ho Lee, Jennifer L. Silhavy, Ji Eun Lee, Lihadh Al-Gazali, Sophie Thomas, Erica E. Davis, Stephanie L. Bielas, Kiley J. Hill, Miriam Iannicelli, Francesco Brancati, Stacey B. Gabriel, Carsten Russ, Clare V. Logan, Saghira Malik Sharif, Christopher P. Bennett, Masumi Abe, Friedhelm Hildebrandt, Bill H. Diplas, Tania Attié-Bitach, Nicholas Katsanis, Anna Rajab, Roshan Koul, Laszlo Sztriha, Elizabeth R. Waters, Susan Ferro-Novick, C. Geoffrey Woods, Colin A. Johnson, Enza Maria Valente, Maha S. Zaki, Joseph G. Gleeson
[Research Article] Evolution of Shape by Multiple Regulatory Changes to a Growth Gene
Authors: David W. Loehlin, John H. Werren
Ten Years of Pathway Analysis: Current Approaches and Outstanding Challenges
by Purvesh Khatri, Marina Sirota, Atul J. Butte
Pathway analysis has become the first choice for gaining insight into the underlying biology of differentially expressed genes and proteins, as it reduces complexity and has increased explanatory power. We discuss the evolution of knowledge base–driven pathway analysis over its first decade, distinctly divided into three generations. We also discuss the limitations that are specific to each generation, and how they are addressed by successive generations of methods. We identify a number of annotation challenges that must be addressed to enable development of the next generation of pathway analysis methods. Furthermore, we identify a number of methodological challenges that the next generation of methods must tackle to take advantage of the technological advances in genomics and proteomics in order to improve specificity, sensitivity, and relevance of pathway analysis.
Wednesday, February 22, 2012
Histone Deacetylase Complexes Promote Trinucleotide Repeat Expansions
by Kim Debacker, Aisling Frizzell, Olive Gleeson, Lucy Kirkham-McCarthy, Tony Mertz, Robert S. Lahue
Expansions of DNA trinucleotide repeats cause at least 17 inherited neurodegenerative diseases, such as Huntington's disease. Expansions can occur at frequencies approaching 100% in affected families and in transgenic mice, suggesting that specific cellular proteins actively promote (favor) expansions. The inference is that expansions arise due to the presence of these promoting proteins, not their absence, and that interfering with these proteins can suppress expansions. The goal of this study was to identify novel factors that promote expansions. We discovered that specific histone deacetylase complexes (HDACs) promote CTG•CAG repeat expansions in budding yeast and human cells. Mutation or inhibition of yeast Rpd3L or Hda1 suppressed up to 90% of expansions. In cultured human astrocytes, expansions were suppressed by 75% upon inhibition or knockdown of HDAC3, whereas siRNA against the histone acetyltransferases CBP/p300 stimulated expansions. Genetic and molecular analysis both indicated that HDACs act at a distance from the triplet repeat to promote expansions. Expansion assays with nuclease mutants indicated that Sae2 is one of the relevant factors regulated by Rpd3L and Hda1. The causal relationship between HDACs and expansions indicates that HDACs can promote mutagenesis at some DNA sequences. This relationship further implies that HDAC3 inhibitors being tested for relief of expansion-associated gene silencing may also suppress somatic expansions that contribute to disease progression.
Tuesday, February 21, 2012
Dorsal-Ventral Gene Expression in the Drosophila Embryo Reflects the Dynamics and Precision of the Dorsal Nuclear Gradient
Molecular mechanism underlying the regulatory specificity of a Drosophila homeodomain protein that specifies myoblast identity [RESEARCH ARTICLES]
A subfamily of Drosophila homeodomain (HD) transcription factors (TFs) controls the identities of individual muscle founder cells (FCs). However, the molecular mechanisms by which these TFs generate unique FC genetic programs remain unknown. To investigate this problem, we first applied genome-wide mRNA expression profiling to identify genes that are activated or repressed by the muscle HD TFs Slouch (Slou) and Muscle segment homeobox (Msh). Next, we used protein-binding microarrays to define the sequences that are bound by Slou, Msh and other HD TFs that have mesodermal expression. These studies revealed that a large class of HDs, including Slou and Msh, predominantly recognize TAAT core sequences but that each HD also binds to unique sites that deviate from this canonical motif. To understand better the regulatory specificity of an individual FC identity HD, we evaluated the functions of atypical binding sites that are preferentially bound by Slou relative to other HDs within muscle enhancers that are either activated or repressed by this TF. These studies showed that Slou regulates the activities of particular myoblast enhancers through Slou-preferred sequences, whereas swapping these sequences for sites that are capable of binding to multiple HD family members does not support the normal regulatory functions of Slou. Moreover, atypical Slou-binding sites are overrepresented in putative enhancers associated with additional Slou-responsive FC genes. Collectively, these studies provide new insights into the roles of individual HD TFs in determining cellular identity, and suggest that the diversity of HD binding preferences can confer regulatory specificity.
The Majority of Animal Genes Are Required for Wild-Type Fitness
Mechanism of Transcription Initiation at an Activator-Dependent Promoter Defined by Single-Molecule Observation
Spatial Organization of the Mouse Genome and Its Role in Recurrent Chromosomal Translocations
Thursday, February 16, 2012
A Conserved Transcriptional Regulator Governs Fungal Morphology in Widely Diverged Species [Gene Expression]
Fungi exhibit a large variety of morphological forms. Here, we examine the functions of a deeply conserved regulator of morphology in three fungal species: Saccharomyces cerevisiae, Candida albicans, and Histoplasma capsulatum. We show that, despite an estimated 600 million years since those species diverged from a common ancestor, Wor1 in C. albicans, Ryp1 in H. capsulatum, and Mit1 in S. cerevisiae are transcriptional regulators that recognize the same DNA sequence. Previous work established that Wor1 regulates white–opaque switching in C. albicans and that its ortholog Ryp1 regulates the yeast to mycelial transition in H. capsulatum. Here we show that the ortholog Mit1 in S. cerevisiae is also a master regulator of a morphological transition, in this case pseudohyphal growth. Full-genome chromatin immunoprecipitation experiments show that Mit1 binds to the control regions of the previously known regulators of pseudohyphal growth as well as those of many additional genes. Through a comparison of binding sites for Mit1 in S. cerevisiae, Wor1 in C. albicans, and Wor1 ectopically expressed in S. cerevisiae, we conclude that the genes controlled by the orthologous regulators overlap only slightly between these two species despite the fact that the DNA binding specificity of the regulators has remained largely unchanged. We suggest that the ancestral Wor1/Mit1/Ryp1 protein controlled aspects of cell morphology and that movement of genes in and out of the Wor1/Mit1/Ryp1 regulon is responsible, in part, for the differences of morphological forms among these species.
Wednesday, February 15, 2012
Single-molecule imaging of DNA pairing by RecA reveals a three-dimensional homology
search
Single-molecule imaging of DNA pairing by RecA reveals a three-dimensional homology
search
Nature 482, 7385 (2012). doi:10.1038/nature10782
Authors: Anthony L. Forget & Stephen C. Kowalczykowski
DNA breaks can be repaired with high fidelity by homologous recombination. A
ubiquitous protein that is essential for this DNA template-directed repair is
RecA. After resection of broken DNA to produce single-stranded
DNA (ssDNA), RecA assembles on this ssDNA into a filament with the unique capacity
to search and find DNA sequences in double-stranded DNA (dsDNA) that are homologous
to the ssDNA. This homology search is vital to recombinational DNA repair, and
results in homologous pairing and exchange of DNA strands. Homologous pairing
involves DNA sequence-specific target location by the RecA–ssDNA complex.
Despite decades of study, the mechanism of this enigmatic search process remains
unknown. RecA is a DNA-dependent ATPase, but ATP hydrolysis is not required for DNA
pairing and strand exchange, eliminating active search processes.
Using dual optical trapping to manipulate DNA, and single-molecule fluorescence
microscopy to image DNA pairing, we demonstrate that both the three-dimensional
conformational state of the dsDNA target and the length of the homologous
RecA–ssDNA filament have important roles in the homology search. We
discovered that as the end-to-end distance of the target dsDNA molecule is
increased, constraining the available three-dimensional (3D) conformations of the
molecule, the rate of homologous pairing decreases. Conversely, when the length of
the ssDNA in the nucleoprotein filament is increased, homology is found faster. We
propose a model for the DNA homology search process termed ‘intersegmental
contact sampling’, in which the intrinsic multivalent nature of the RecA
nucleoprotein filament is used to search DNA sequence space within 3D domains of
DNA, exploiting multiple weak contacts to rapidly search for homology. Our findings
highlight the importance of the 3D conformational dynamics of DNA, reveal a
previously unknown facet of the homology search, and provide insight into the
mechanism of DNA target location by this member of a universal family of
proteins.
Tuesday, February 14, 2012
Evolution and Function of the Globin Intergenic Regulatory Regions of the Antarctic Dragonfishes (Notothenioidei: Bathydraconidae)
As the Southern Ocean cooled to –1.8 °C over the past 40 My, the teleostean clade Notothenioidei diversified and, under reduced selection pressure for an oxygen-transporting apparatus, became less reliant on hemoglobin and red blood cells. At the extreme of this trend, the crown group of Antarctic icefishes (Channichthyidae) lost both components of oxygen transport. Under the decreased selection scenario, we hypothesized that the Antarctic dragonfishes (Bathydraconidae, the red-blooded sister clade to the icefishes) evolved lower blood hemoglobin concentrations because their globin gene complexes (α- and β-globin gene pairs linked by a regulatory intergene) transcribe globin mRNAs less effectively than those of basal notothenioids (e.g., the Nototheniidae [notothens]). To test our hypothesis, we 1) sequenced the α/β-intergenes of the adult globin complexes of three notothen and eight dragonfish species and 2) measured globin transcript levels in representative species from each group. The typical nototheniid intergene was ~3–4 kb in length. The bathydraconid intergenes resolved into three subclasses (long [3.8 kb], intermediate [3.0 kb], and short [1.5–2.3 kb]) that corresponded to the three subclades proposed for the taxon. Although they varied in length due to indels, the three notothen and eight dragonfish intergenes contained a conserved ~90-nt element that we have previously shown to be required for globin gene transcription. Using the quantitative polymerase chain reaction, we found that globin mRNA levels in red cells from one notothen species and from one species of each dragonfish subclade were equivalent statistically. Thus, our results indicate that the bathydraconids have evolved adult globin loci whose regulatory intergenes tend to be shorter than those of the more basal nototheniids yet are equivalent in transcriptional efficacy. Their low blood hemoglobin concentrations are most likely due to reduction in hematocrit.
Evolutionary Origins of Transcription Factor Binding Site Clusters
Empirical studies have revealed that regulatory DNA sequences such as enhancers or promoters often harbor multiple binding sites for the same transcription factor. Such "homotypic site clustering" has been hypothesized as arising out of functional requirements of the sequences. Here, we propose an alternative explanation of this phenomenon that multisite enhancers are common because they are favored by evolutionary sampling of the genotype–phenotype landscape. To test this hypothesis, we developed a new computational framework specialized for population genetic simulations of enhancer evolution. It uses a thermodynamics-based model of enhancer function, integrating information from strong as well as weak binding sites, to determine the strength of selection. Using this framework, we found that even when simpler genotypes exist for a desired strength of regulation, relatively complex genotypes (enhancers with more sites) are more readily reached by the simulated evolutionary process. We show that there are more ways to "build" a fit genotype with many weak sites than with a few strong sites, and this is why evolution finds complex genotypes more often. Our claims are consistent with an empirical analysis of binding site content in enhancers characterized in Drosophila melanogaster and their orthologs in other Drosophila species. We also characterized a subtle but significant difference between genotypes likely to be sampled by evolution and equally fit genotypes one would obtain by uniform sampling of the fitness landscape, that is, an "evolutionary signature" in enhancer sequences. Finally, we investigated potential effects of other factors, such as rugged fitness landscapes, short local duplications, and noise characteristics of enhancers, on the emergence of homotypic site clustering.
Homotypic site clustering is an important contributor to the complexity and function of cis-regulatory sequences. This work provides a simple null hypothesis for its origin, against which alternative adaptationist explanations may be evaluated, and cautions against "evolutionary mirages" present in common features of genomic sequence. The quantitative framework we develop here can be used more generally to understand how mechanisms of enhancer action influence their composition and evolution.
A Living Fossil in the Genome of a Living Fossil: Harbinger Transposons in the Coelacanth Genome
Emerging data from the coelacanth genome are beginning to shed light on the origin and evolution of tetrapod genes and noncoding elements. Of particular relevance is the realization that coelacanth retains active copies of transposable elements that once served as raw material for the evolution of new functional sequences in the vertebrate lineage. Recognizing the evolutionary significance of coelacanth genome in this regard, we employed an ab initio search strategy to further classify its repetitive complement. This analysis uncovered a class of interspersed elements (Latimeria Harbinger 1—LatiHarb1) that is a major contributor to coelacanth genome structure and gene content (~1% to 4% or the genome). Sequence analyses indicate that 1) each ~8.7 kb LatiHarb1 element contains two coding regions, a transposase gene and a gene whose function is as yet unknown (MYB-like) and 2) copies of LatiHarb1 retain biological activity in the coelacanth genome. Functional analyses verify transcriptional and enhancer activities of LatiHarb1 in vivo and reveal transcriptional decoupling that could permit MYB-like genes to play functional roles not directly linked to transposition. Thus, LatiHarb1 represents the first known instance of a harbinger-superfamily transposon with contemporary activity in a vertebrate genome. Analyses of LatiHarb1 further corroborate the notion that exaptation of anciently active harbinger elements gave rise to at least two vertebrate genes (harbi1 and naif1) and indicate that the vertebrate gene tsnare1 also traces its ancestry to this transposon superfamily. Based on our analyses of LatiHarb1, we speculate that several functional features of harbinger elements may predispose the transposon superfamily toward recurrent exaptive evolution of cellular coding genes. In addition, these analyses further reinforce the broad utility of the coelacanth genome and other "outgroup" genomes in understanding the ancestry and evolution of vertebrate genes and genomes.
Determinants of the Drosophila Odorant Receptor Pattern
Friday, February 10, 2012
A Regulatory Network for Coordinated Flower Maturation
by Paul H. Reeves, Christine M. Ellis, Sara E. Ploense, Miin-Feng Wu, Vandana Yadav, Dorothea Tholl, Aurore Chételat, Ina Haupt, Brian J. Kennerley, Charles Hodgens, Edward E. Farmer, Punita Nagpal, Jason W. Reed
For self-pollinating plants to reproduce, male and female organ development must be coordinated as flowers mature. The Arabidopsis transcription factors AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8 regulate this complex process by promoting petal expansion, stamen filament elongation, anther dehiscence, and gynoecium maturation, thereby ensuring that pollen released from the anthers is deposited on the stigma of a receptive gynoecium. ARF6 and ARF8 induce jasmonate production, which in turn triggers expression of MYB21 and MYB24, encoding R2R3 MYB transcription factors that promote petal and stamen growth. To understand the dynamics of this flower maturation regulatory network, we have characterized morphological, chemical, and global gene expression phenotypes of arf, myb, and jasmonate pathway mutant flowers. We found that MYB21 and MYB24 promoted not only petal and stamen development but also gynoecium growth. As well as regulating reproductive competence, both the ARF and MYB factors promoted nectary development or function and volatile sesquiterpene production, which may attract insect pollinators and/or repel pathogens. Mutants lacking jasmonate synthesis or response had decreased MYB21 expression and stamen and petal growth at the stage when flowers normally open, but had increased MYB21 expression in petals of older flowers, resulting in renewed and persistent petal expansion at later stages. Both auxin response and jasmonate synthesis promoted positive feedbacks that may ensure rapid petal and stamen growth as flowers open. MYB21 also fed back negatively on expression of jasmonate biosynthesis pathway genes to decrease flower jasmonate level, which correlated with termination of growth after flowers have opened. These dynamic feedbacks may promote timely, coordinated, and transient growth of flower organs.
Thursday, February 9, 2012
Contrasting Properties of Gene-Specific Regulatory, Coding, and Copy Number Mutations in Saccharomyces cerevisiae: Frequency, Effects, and Dominance
by Jonathan D. Gruber, Kara Vogel, Gizem Kalay, Patricia J. Wittkopp
Genetic variation within and between species can be shaped by population-level processes and mutation; however, the relative impact of “survival of the fittest” and “arrival of the fittest” on phenotypic evolution remains unclear. Assessing the influence of mutation on evolution requires understanding the relative rates of different types of mutations and their genetic properties, yet little is known about the functional consequences of new mutations. Here, we examine the spectrum of mutations affecting a focal gene in Saccharomyces cerevisiae by characterizing 231 novel haploid genotypes with altered activity of a fluorescent reporter gene. 7% of these genotypes had a nonsynonymous mutation in the coding sequence for the fluorescent protein and were classified as “coding” mutants; 2% had a change in the S. cerevisiae TDH3 promoter sequence controlling expression of the fluorescent protein and were classified as “cis-regulatory” mutants; 10% contained two copies of the reporter gene and were classified as “copy number” mutants; and the remaining 81% showed altered fluorescence without a change in the reporter gene itself and were classified as “trans-acting” mutants. As a group, coding mutants had the strongest effect on reporter gene activity and always decreased it. By contrast, 50%–95% of the mutants in each of the other three classes increased gene activity, with mutants affecting copy number and cis-regulatory sequences having larger median effects on gene activity than trans-acting mutants. When made heterozygous in diploid cells, coding, cis-regulatory, and copy number mutant genotypes all had significant effects on gene activity, whereas 88% of the trans-acting mutants appeared to be recessive. These differences in the frequency, effects, and dominance among functional classes of mutations might help explain why some types of mutations are found to be segregating within or fixed between species more often than others.
Rewiring of PDZ Domain-Ligand Interaction Network Contributed to Eukaryotic Evolution
by Jinho Kim, Inhae Kim, Jae-Seong Yang, Young-Eun Shin, Jihye Hwang, Solip Park, Yoon Sup Choi, Sanguk Kim
PDZ domain-mediated interactions have greatly expanded during metazoan evolution, becoming important for controlling signal flow via the assembly of multiple signaling components. The evolutionary history of PDZ domain-mediated interactions has never been explored at the molecular level. It is of great interest to understand how PDZ domain-ligand interactions emerged and how they become rewired during evolution. Here, we constructed the first human PDZ domain-ligand interaction network (PDZNet) together with binding motif sequences and interaction strengths of ligands. PDZNet includes 1,213 interactions between 97 human PDZ proteins and 591 ligands that connect most PDZ protein-mediated interactions (98%) in a large single network via shared ligands. We examined the rewiring of PDZ domain-ligand interactions throughout eukaryotic evolution by tracing changes in the C-terminal binding motif sequences of the PDZ ligands. We found that interaction rewiring by sequence mutation frequently occurred throughout evolution, largely contributing to the growth of PDZNet. The rewiring of PDZ domain-ligand interactions provided an effective means of functional innovations in nervous system development. Our findings provide empirical evidence for a network evolution model that highlights the rewiring of interactions as a mechanism for the development of new protein functions. PDZNet will be a valuable resource to further characterize the organization of the PDZ domain-mediated signaling proteome.
Fossil beetle colour
Structural colours, the most intense, reflective and pure colours in nature, are generated when light is scattered by complex nanostructures. Metallic structural colours are widespread among modern insects and can be preserved in their fossil counterparts, but it is unclear whether the colours have been altered during fossilization, and whether the absence of colours is always real. To resolve these issues, we investigated fossil beetles from five Cenozoic biotas. Metallic colours in these specimens are generated by an epicuticular multi-layer reflector; the fidelity of its preservation correlates with that of other key cuticular ultrastructures. Where these other ultrastructures are well preserved in non-metallic fossil specimens, we can infer that the original cuticle lacked a multi-layer reflector; its absence in the fossil is not a preservational artefact. Reconstructions of the original colours of the fossils based on the structure of the multi-layer reflector show that the preserved colours are offset systematically to longer wavelengths; this probably reflects alteration of the refractive index of the epicuticle during fossilization. These findings will allow the former presence, and original hue, of metallic structural colours to be identified in diverse fossil insects, thus providing critical evidence of the evolution of structural colour in this group.
Wednesday, February 8, 2012
Enhancer decommissioning by LSD1 during embryonic stem cell differentiation
Enhancer decommissioning by LSD1 during embryonic stem cell differentiation
Nature 482, 7384 (2012). doi:10.1038/nature10805
Authors: Warren A. Whyte, Steve Bilodeau, David A. Orlando, Heather A. Hoke, Garrett M. Frampton, Charles T. Foster, Shaun M. Cowley & Richard A. Young
Transcription factors and chromatin modifiers are important in the programming and reprogramming of cellular states during development. Transcription factors bind to enhancer elements and recruit coactivators and chromatin-modifying enzymes to facilitate transcription initiation. During differentiation a subset of these enhancers must be silenced, but the mechanisms underlying enhancer silencing are poorly understood. Here we show that the histone demethylase lysine-specific demethylase 1 (LSD1; ref. 5), which demethylates histone H3 on Lys 4 or Lys 9 (H3K4/K9), is essential in decommissioning enhancers during the differentiation of mouse embryonic stem cells (ESCs). LSD1 occupies enhancers of active genes that are critical for control of the state of ESCs. However, LSD1 is not essential for the maintenance of ESC identity. Instead, ESCs lacking LSD1 activity fail to differentiate fully, and ESC-specific enhancers fail to undergo the histone demethylation events associated with differentiation. At active enhancers, LSD1 is a component of the NuRD (nucleosome remodelling and histone deacetylase) complex, which contains additional subunits that are necessary for ESC differentiation. We propose that the LSD1–NuRD complex decommissions enhancers of the pluripotency program during differentiation, which is essential for the complete shutdown of the ESC gene expression program and the transition to new cell states.
Tuesday, February 7, 2012
A function for all posterior HoxD genes during digit development?
Abstract
Background.
Four posterior Hoxd genes, from Hoxd13 to Hoxd10, are collectively regulated during the development of tetrapod digits. Besides the well-documented role of Hoxd13, the function of the neighboring genes has been difficult to evaluate due to the close genetic linkage and potential regulatory interferences. We used a panel of five small nested deletions in cis, involving from two to four consecutive genes, of the Hoxd13 to Hoxd9 loci, in mice, to evaluate their combined functional importance.
Results.
We show that deletions leading to a gain of function of Hoxd13, via regulatory re-allocation, generate abnormal phenotypes, in agreement with the dominant negative role of this gene. We also show that Hoxd10, Hoxd11 and Hoxd12 all seem to play a genuine role in digit development, though less compelling than that of Hoxd13. In contrast, the nearby Hoxd9 contributed no measurable function in digits.
Conclusions.
We conclude that a slight and transient deregulation of Hoxd13 expression can readily affect the relative lengths of limb segments and that all posterior Hoxd genes likely contribute to the final limb morphology. We discuss the difficulty to clearly assess the functional share of individual genes within such a gene family, where closely located neighbors, coding for homologous proteins, are regulated by a unique circuitry and all contribute to shape the distal parts of our appendages. Developmental Dynamics, 2012. © 2012 Wiley Periodicals, Inc.
Bimodal control of Hoxd gene transcription in the spinal cord defines two regulatory subclusters [RESEARCH ARTICLES]
The importance of Hox genes in the specification of neuronal fates in the spinal cord has long been recognized. However, the transcriptional controls underlying their collinear expression domains remain largely unknown. Here we show in mice that the correspondence between the physical order of Hoxd genes and their rostral expression boundaries, although respecting spatial collinearity, does not display a fully progressive distribution. Instead, two major anteroposterior boundaries are detected, coinciding with the functional subdivision of the spinal cord. Tiling array analyses reveal two distinct blocks of transcription, regulated independently from one another, that define the observed expression boundaries. Targeted deletions in vivo that remove the genomic fragments separating the two blocks induce ectopic expression of posterior genes. We further evaluate the independent regulatory potential and transcription profile of each gene locus by a tiling array approach using a contiguous series of transgenes combined with locus-specific deletions. Our work uncovers a bimodal type of HoxD spatial collinearity in the developing spinal cord that relies on two separate ‘enhancer mini-hubs’ to ensure correct Hoxd gene expression levels while maintaining their appropriate anteroposterior boundaries.
Cis-regulatory properties of medaka synexpression groups [RESEARCH ARTICLES]
During embryogenesis, tissue specification is triggered by the expression of a unique combination of developmental genes and their expression in time and space is crucial for successful development. Synexpression groups are batteries of spatiotemporally co-expressed genes that act in shared biological processes through their coordinated expression. Although several synexpression groups have been described in numerous vertebrate species, the regulatory mechanisms that orchestrate their common complex expression pattern remain to be elucidated. Here we performed a pilot screen on 560 genes of the vertebrate model system medaka (Oryzias latipes) to systematically identify synexpression groups and investigate their regulatory properties by searching for common regulatory cues. We find that synexpression groups share DNA motifs that are arranged in various combinations into cis-regulatory modules that drive co-expression. In contrast to previous assumptions that these genes are located randomly in the genome, we discovered that genes belonging to the same synexpression group frequently occur in synexpression clusters in the genome. This work presents a first repertoire of synexpression group common signatures, a resource that will contribute to deciphering developmental gene regulatory networks.
Friday, February 3, 2012
Evaluation of the Role of Functional Constraints on the Integrity of an Ultraconserved Region in the Genus Drosophila
by Carlos Díaz-Castillo, Xiao-Qin Xia, José M. Ranz
Why gene order is conserved over long evolutionary timespans remains elusive. A common interpretation is that gene order conservation might reflect the existence of functional constraints that are important for organismal performance. Alteration of the integrity of genomic regions, and therefore of those constraints, would result in detrimental effects. This notion seems especially plausible in those genomes that can easily accommodate gene reshuffling via chromosomal inversions since genomic regions free of constraints are likely to have been disrupted in one or more lineages. Nevertheless, no empirical test has been performed to this notion. Here, we disrupt one of the largest conserved genomic regions of the Drosophila genome by chromosome engineering and examine the phenotypic consequences derived from such disruption. The targeted region exhibits multiple patterns of functional enrichment suggestive of the presence of constraints. The carriers of the disrupted collinear block show no defects in their viability, fertility, and parameters of general homeostasis, although their odorant perception is altered. This change in odorant perception does not correlate with modifications of the level of expression and sex bias of the genes within the genomic region disrupted. Our results indicate that even in highly rearranged genomes, like those of Diptera, unusually high levels of gene order conservation cannot be systematically attributed to functional constraints, which raises the possibility that other mechanisms can be in place and therefore the underpinnings of the maintenance of gene organization might be more diverse than previously thought.
SEXUAL SELECTION IS INEFFECTUAL OR INHIBITS THE PURGING OF DELETERIOUS MUTATIONS IN DROSOPHILA MELANOGASTER
ABSTRACT
The effects of sexual selection on population mean fitness are unclear and a subject of debate. Recent models propose that, because reproductive success may be condition-dependent, much of the genome may be a target of sexual selection. Under this scenario, mutations that reduce health, and thus non-sexual fitness, may also be deleterious with respect to reproductive success, meaning that sexual selection may contribute to the purging of deleterious alleles. We tested this hypothesis directly by subjecting replicate Drosophila melanogaster populations to two treatments that altered the opportunity for sexual selection and then tracked changes in the frequency of six separate deleterious alleles with recessive and visible phenotypic effects. While natural selection acted to decrease the frequency of all six mutations, the addition of sexual selection did not aid in the purging of any of them, and for three of them appears to have hampered it. Courtship and mating have harmful effects in this species and mate choice assays showed that males directed more courtship and mating behavior towards wild-type over mutant females, providing a likely explanation for sexual selection's cost. Whether this cost extends to other mutations (e.g., those lacking visible phenotypic effects) is an important topic for future research.
2012 The Author(s). Journal compilation © 2012 The Society for the Study of Evolution
[Report] Sequential Signaling Crosstalk Regulates Endomesoderm Segregation in Sea Urchin Embryos
Authors: Aditya J. Sethi, Radhika M. Wikramanayake, Robert C. Angerer, Ryan C. Range, Lynne M. Angerer
[Report] Natural Variation in a Chloride Channel Subunit Confers Avermectin Resistance in C. elegans
Authors: Rajarshi Ghosh, Erik C. Andersen, Joshua A. Shapiro, Justin P. Gerke, Leonid Kruglyak